Tomographic imaging of oxygen by phosphorescence lifetime

Simultaneous Probing of Metabolism and Oxygenation of Tumors In Vivo Using FLIM of NAD(P)H and PLIM of a New Polymeric Ir(III) Oxygen Sensor

Tumor cells are well adapted to grow in conditions of variable oxygen supply and hypoxia by switching between different metabolic pathways. However, the regulatory effect of oxygen on metabolism and its contribution to the metabolic heterogeneity of tumors have not been fully explored. In this study, we develop a methodology for the simultaneous analysis of cellular metabolic status, using the fluorescence lifetime imaging microscopy (FLIM) of metabolic cofactor NAD(P)H, and oxygen level, using the phosphorescence lifetime imaging (PLIM) of a new polymeric Ir(III)-based sensor (PIr3) in tumors in vivo. The sensor, derived from a polynorbornene and cyclometalated iridium(III) complex, exhibits the oxygen-dependent quenching of phosphorescence with a 40% longer lifetime in degassed compared to aerated solutions. In vitro, hypoxia resulted in a correlative increase in PIr3 phosphorescence lifetime and free (glycolytic) NAD(P)H fraction in cells. In vivo, mouse tumors demonstrated a high degree of cellular-level heterogeneity of both metabolic and oxygen states, and a lower dependence of metabolism on oxygen than cells in vitro. The small tumors were hypoxic, while the advanced tumors contained areas of normoxia and hypoxia, which was consistent with the pimonidazole assay and angiographic imaging. Dual FLIM/PLIM metabolic/oxygen imaging will be valuable in preclinical investigations into the effects of hypoxia on metabolic aspects of tumor progression and treatment response.

Biological validation of electron paramagnetic resonance (EPR) image oxygen thresholds in tissue

Measuring molecular oxygen levels in vivo has been the cornerstone of understanding the effects of hypoxia in normal tissues and malignant tumors. Here we discuss the advances in a variety of partial pressure of oxygen ( P O 2 ) measurements and imaging techniques and relevant oxygen thresholds. A focus on electron paramagnetic resonance (EPR) imaging shows the validation of treating hypoxic tumours with a threshold of P O 2  ≤ 10 Torr, and demonstrates utility for in vivo oxygen imaging, as well as its current and future role in cancer studies.

Mapping O2 concentration in ex-vivo tissue samples on a fast PLIM macro-imager

O2 PLIM microscopy was employed in various studies, however current platforms have limitations in sensitivity, image acquisition speed, accuracy and general usability. We describe a new PLIM imager based on the Timepix3 camera (Tpx3cam) and its application for imaging of O2 concentration in various tissue samples stained with a nanoparticle based probe, NanO2-IR. Upon passive staining of mouse brain, lung or intestinal tissue surface with minute quantities of NanO2-IR or by microinjecting the probe into the lumen of small or large intestine fragments, robust phosphorescence intensity and lifetime signals were produced, which allow mapping of O2 in the tissue within 20 s. Inhibition of tissue respiration or limitation of O2 diffusion to tissue produced the anticipated increases or decreases in O2 levels, respectively. The difference in O2 concentration between the colonic lumen and air-exposed serosal surface was around 140 µM. Furthermore, subcutaneous injection of 5 µg of the probe in intact organs (a paw or tail of sacrificed mice) enabled efficient O2 imaging at tissue depths of up to 0.5 mm. Overall, the PLIM imager holds promise for metabolic imaging studies with various ex vivo models of animal tissue, and also for use in live animals.

A Synthetic Approach to β-Functionalized Naphtho[2,3]porphyrins

A concise synthetic method has been developed to access functionalized naphtho[2,3]porphyrins through combining two sequence reactions involving a Heck-electrocyclization-aromatization sequence and a Wittig-Knovenegal sequence. Using this method, mononaphtho[2,3]porphyrin (NP-1), opp-dinaphtho[2,3]porphyrin (NP-2), and push-pull naphtho[2,3]porphyrin (NP-3) have been prepared. These naphtho[2,3]porphyrins displayed interesting optical and electrochemical properties. Excellent efficiencies of singlet oxygen generation were obtained for these naphtho[2,3]porphyrins.

Renal tissue Po2sensing during acute hemodilution is dependent on the diluent

Sensing changes in blood oxygen content ([Formula: see text]) is an important physiological role of the kidney; however, the mechanism(s) by which the kidneys sense and respond to changes in [Formula: see text] are incompletely understood. Accurate measurements of kidney tissue oxygen tension ([Formula: see text]) may increase our understanding of renal oxygen-sensing mechanisms and could inform decisions regarding the optimal fluid for intravascular volume resuscitation to maintain renal perfusion. In some clinical settings, starch solution may be nephrotoxic, possibly due to inadequacy of tissue oxygen delivery. We hypothesized that hemodilution with starch colloid solutions would reduce [Formula: see text] to a more severe degree than other diluents. Anesthetized Sprague-Dawley rats ( n = 77) were randomized to undergo hemodilution with either colloid (6% hydroxyethyl starch or 5% albumin), crystalloid (0.9% saline), or a sham procedure (control) ( n = 13–18 rats/group). Data were analyzed by ANOVA with significance assigned at P < 0.05. After hemodilution, mean arterial pressure (MAP) decreased marginally in all groups, while hemoglobin (Hb) and [Formula: see text] decreased in proportion to the degree of hemodilution. Cardiac output was maintained in all groups after hemodilution. [Formula: see text] decreased in proportion to the reduction in Hb in all treatment groups. At comparably reduced Hb, and maintained arterial oxygen values, hemodilution with starch resulted in larger decreases in [Formula: see text] relative to animals hemodiluted with albumin or saline ( P < 0.008). Renal medullary erythropoietin (EPO) mRNA levels increased more prominently, relative to other hypoxia-regulated molecules (GLUT-1, GAPDH, and VEGF). Our data demonstrate that the kidney acts as a biosensor of reduced [Formula: see text] following hemodilution and that [Formula: see text] may provide a quantitative signal for renal cellular responsiveness to acute anemia. Evidence of a more severe reduction in [Formula: see text] following hemodilution with starch colloid solution suggests that tissue hypoxia may contribute to starch induced renal toxicity.

Tissue pO2 distributions in xenograft tumors dynamically imaged by Cherenkov-excited phosphorescence during fractionated radiation therapy

Hypoxia in solid tumors is thought to be an important factor in resistance to therapy, but the extreme microscopic heterogeneity of the partial pressures of oxygen (pO₂) between the capillaries makes it difficult to characterize the scope of this phenomenon without invasive sampling of oxygen distributions throughout the tissue. Here we develop a non-invasive method to track spatial oxygen distributions in tumors during fractionated radiotherapy, using oxygen-dependent quenching of phosphorescence, oxygen probe Oxyphor PtG4 and the radiotherapy-induced Cherenkov light to excite and image the phosphorescence lifetimes within the tissue. Mice bearing MDA-MB-231 breast cancer and FaDu head neck cancer xenografts show different pO₂ responses during each of the 5 fractions (5 Gy per fraction), delivered from a clinical linear accelerator. This study demonstrates subsurface in vivo mapping of tumor pO₂ distributions with submillimeter spatial resolution, thus providing a methodology to track response of tumors to fractionated radiotherapy.

Multi-beam scan analysis with a clinical LINAC for high resolution Cherenkov-excited molecular luminescence imaging in tissue

Cherenkov-excited luminescence scanned imaging (CELSI) is achieved with external beam radiotherapy to map out molecular luminescence intensity or lifetime in tissue. Just as in fluorescence microscopy, the choice of excitation geometry can affect the imaging time, spatial resolution and contrast recovered. In this study, the use of spatially patterned illumination was systematically studied comparing scan shapes, starting with line scan and block patterns and increasing from single beams to multiple parallel beams and then to clinically used treatment plans for radiation therapy. The image recovery was improved by a spatial-temporal modulation-demodulation method, which used the ability to capture simultaneous images of the excitation Cherenkov beam shape to deconvolve the CELSI images. Experimental studies used the multi-leaf collimator on a clinical linear accelerator (LINAC) to create the scanning patterns, and image resolution and contrast recovery were tested at different depths of tissue phantom material. As hypothesized, the smallest illumination squares achieved optimal resolution, but at the cost of lower signal and slower imaging time. Having larger excitation blocks provided superior signal but at the cost of increased radiation dose and lower resolution. Increasing the scan beams to multiple block patterns improved the performance in terms of image fidelity, lower radiation dose and faster acquisition. The spatial resolution was mostly dependent upon pixel area with an optimized side length near 38mm and a beam scan pitch of P = 0.33, and the achievable imaging depth was increased from 14mm to 18mm with sufficient resolving power for 1mm sized test objects. As a proof-of-concept, in-vivo tumor mouse imaging was performed to show 3D rendering and quantification of tissue pO2 with values of 5.6mmHg in a tumor and 77mmHg in normal tissue.

Tomographic phosphorescence lifetime multiplexing

We present a tomographic reconstruction algorithm for recovering distributions of multiple phosphorescent dyes within turbid media from time-resolved measurements, using either point or spatially patterned sources and detectors. The algorithm employs a multi-exponential analysis of time-resolved data, followed by tomographic inversion of the decay amplitudes to recover independent yield distributions for each lifetime present in the medium. Using Monte Carlo simulations, we computationally demonstrate that this two-step inversion approach provides several-fold improvement in quantitative and localization accuracy compared to a direct inversion of the time domain phosphorescence. We also demonstrate the tomographic reconstruction of up to three phosphorescent lifetimes embedded in thick tissue. The proposed algorithm can allow quantitative multiplexed tomography of luminescent and phosphorescent dyes for a wide range of in vivo applications.

Water-soluble cyclometalated platinum(ii) and iridium(iii) complexes: synthesis, tuning of the photophysical properties, and in vitro and in vivo phosphorescence lifetime imaging

This paper presents synthesis and photophysical investigation of cyclometalated water-soluble Pt(ii) and Ir(iii) complexes containing auxiliary sulfonated diphosphine (bis(diphenylphosphino)benzene (dppb), P^P*) ligand. The complexes demonstrate considerable variations in excitation (extending up to 450 nm) and emission bands (with maxima ranging from ca. 450 to ca. 650 nm), as well as in the sensitivity of excited state lifetimes to molecular oxygen (from almost negligible to more than 4-fold increase in degassed solution). Moreover, all the complexes possess high two-photon absorption cross sections (400–500 GM for Pt complexes, and 600–700 GM for Ir complexes). Despite their negative net charge, all the complexes demonstrate good uptake by HeLa cells and low cytotoxicity within the concentration and time ranges suitable for two-photon phosphorescence lifetime (PLIM) microscopy. The most promising complex, [(ppy)₂Ir(sulfo-dppb)] (Ir1*), upon incubation in HeLa cells demonstrates two-fold lifetime variations under normal and nitrogen atmosphere, correspondingly. Moreover, its in vivo evaluation in athymic nude mice bearing HeLa tumors did not reveal acute toxicity upon both intravenous and topical injections. Finally, Ir1* demonstrated statistically significant difference in lifetimes between normal tissue (muscle) and tumor in macroscopic in vivo PLIM imaging.

Novel water-soluble iridium complexes with sulfonated diphosphine allow in vitro and in vivo lifetime hypoxia imaging.

Quantitative monitoring of the level of singlet oxygen using luminescence spectra of phosphorescent photosensitizer

The lack of accurate and robust photodynamic therapy dosimetry is one of the obstacles for the widespread clinical applications. In this study, we propose a methodology to monitor the production of reactive oxygen species in real-time using the phosphorescent spectra of metalloporphyrin based photosensitizer. The correlation among the phosphorescence intensity, the (1)O(2) quantum yield (Φ(Δ)) and the oxygen concentration [O(2)] was established. A method of determining Φ(Δ) with different [O(2)] was studied based on comparative spectrophotometry, and the quantum yield Φ(Δ) of gadolinium metalated hematoporphyrin mono ether (Gd-HMME) in methanol was determined for different [O(2)]. With our method, both [O(2)] and Φ(Δ) could be monitored simultaneously using the phosphorescence spectra. The photochemical reactions in a liquid phantom composed of Gd-HMME and (1)O(2) capture 1,3-diphenylisobenzofuran (DPBF) were correlated using the kinetics equations of singlet oxygen generation and reaction. Using our method, the (1)O(2) quantum yield becomes observable and the (1)O(2) dose rate could be calculated by the product of photosensitizer absorption and its (1)O(2) quantum yield. Moreover, this (1)O(2) dosimetry could be observed by spectral imaging intuitively without complex analysis, and is especially suitable for precise customized photodynamic treatment.

Oxygenation measurement by multi-wavelength oxygen-dependent phosphorescence and delayed fluorescence: catchment depth and application in intact heart

Oxygen delivery and metabolism represent key factors for organ function in health and disease. We describe the optical key characteristics of a technique to comprehensively measure oxygen tension (PO(2)) in myocardium, using oxygen-dependent quenching of phosphorescence and delayed fluorescence of porphyrins, by means of Monte Carlo simulations and ex vivo experiments. Oxyphor G2 (microvascular PO(2)) was excited at 442 nm and 632 nm and protoporphyrin IX (mitochondrial PO(2)) at 510 nm. This resulted in catchment depths of 161 (86) µm, 350 (307) µm and 262 (255) µm respectively, as estimated by Monte Carlo simulations and ex vivo experiments (brackets). The feasibility to detect changes in oxygenation within separate anatomical compartments is demonstrated in rat heart in vivo. Schematic of ex vivo measurements.

Imaging oxygen in neural cell and tissue models by means of anionic cell-permeable phosphorescent nanoparticles

Cell-permeable phosphorescent probes enable the study of cell and tissue oxygenation, bioenergetics, metabolism, and pathological states such as stroke and hypoxia. A number of such probes have been described in recent years, the majority consisting of cationic small molecule and nanoparticle structures. While these probes continue to advance, adequate staining for the study of certain cell types using live imaging techniques remains elusive; this is particularly true for neural cells. Here we introduce novel probes for the analysis of neural cells and tissues: negatively charged poly(methyl methacrylate-co-methacrylic acid)-based nanoparticles impregnated with a phosphorescent Pt(II)-tetrakis(pentafluorophenyl)porphyrin (PtPFPP) dye (this form is referred to as PA1), and with an additional reference/antennae dye poly(9,9-diheptylfluorene-alt-9,9-di-p-tolyl-9H-fluorene) (this form is referred to as PA2). PA1 and PA2 are internalised by endocytosis, result in efficient staining in primary neurons, astrocytes, and PC12 cells and multi-cellular aggregates, and allow for the monitoring of local O(2) levels on a time-resolved fluorescence plate reader and PLIM microscope. PA2 also efficiently stains rat brain slices and permits detailed O(2) imaging experiments using both one and two-photon intensity-based modes and PLIM modes. Multiplexed analysis of embryonic rat brain slices reveals age-dependent staining patterns for PA2 and a highly heterogeneous distribution of O(2) in tissues, which we relate to the localisation of specific progenitor cell populations. Overall, these anionic probes are useful for sensing O(2) levels in various cells and tissues, particularly in neural cells, and facilitate high-resolution imaging of O(2) in 3D tissue models.

Correlation between intraluminal oxygen gradient and radial partitioning of intestinal microbiota

BACKGROUND & AIMS

The gut microbiota is a complex and densely populated community in a dynamic environment determined by host physiology. We investigated how intestinal oxygen levels affect the composition of the fecal and mucosally adherent microbiota.

METHODS

We used the phosphorescence quenching method and a specially designed intraluminal oxygen probe to dynamically quantify gut luminal oxygen levels in mice. 16S ribosomal RNA gene sequencing was used to characterize the microbiota in intestines of mice exposed to hyperbaric oxygen, human rectal biopsy and mucosal swab samples, and paired human stool samples.

RESULTS

Average Po2 values in the lumen of the cecum were extremely low (<1 mm Hg). In altering oxygenation of mouse intestines, we observed that oxygen diffused from intestinal tissue and established a radial gradient that extended from the tissue interface into the lumen. Increasing tissue oxygenation with hyperbaric oxygen altered the composition of the gut microbiota in mice. In human beings, 16S ribosomal RNA gene analyses showed an increased proportion of oxygen-tolerant organisms of the Proteobacteria and Actinobacteria phyla associated with rectal mucosa, compared with feces. A consortium of asaccharolytic bacteria of the Firmicute and Bacteroidetes phyla, which primarily metabolize peptones and amino acids, was associated primarily with mucus. This could be owing to the presence of proteinaceous substrates provided by mucus and the shedding of the intestinal epithelium.

CONCLUSIONS

In an analysis of intestinal microbiota of mice and human beings, we observed a radial gradient of microbes linked to the distribution of oxygen and nutrients provided by host tissue.

Non-invasive transdermal two-dimensional mapping of cutaneous oxygenation with a rapid-drying liquid bandage

Oxygen plays an important role in wound healing, as it is essential to biological functions such as cell proliferation, immune responses and collagen synthesis. Poor oxygenation is directly associated with the development of chronic ischemic wounds, which affect more than 6 million people each year in the United States alone at an estimated cost of $25 billion. Knowledge of oxygenation status is also important in the management of burns and skin grafts, as well as in a wide range of skin conditions. Despite the importance of the clinical determination of tissue oxygenation, there is a lack of rapid, user-friendly and quantitative diagnostic tools that allow for non-disruptive, continuous monitoring of oxygen content across large areas of skin and wounds to guide care and therapeutic decisions. In this work, we describe a sensitive, colorimetric, oxygen-sensing paint-on bandage for two-dimensional mapping of tissue oxygenation in skin, burns, and skin grafts. By embedding both an oxygen-sensing porphyrin-dendrimer phosphor and a reference dye in a liquid bandage matrix, we have created a liquid bandage that can be painted onto the skin surface and dries into a thin film that adheres tightly to the skin or wound topology. When captured by a camera-based imaging device, the oxygen-dependent phosphorescence emission of the bandage can be used to quantify and map both the pO2 and oxygen consumption of the underlying tissue. In this proof-of-principle study, we first demonstrate our system on a rat ischemic limb model to show its capabilities in sensing tissue ischemia. It is then tested on both ex vivo and in vivo porcine burn models to monitor the progression of burn injuries. Lastly, the bandage is applied to an in vivo porcine graft model for monitoring the integration of full- and partial-thickness skin grafts.

Cherenkov excited phosphorescence-based pO2 estimation during multi-beam radiation therapy: phantom and simulation studies

Megavoltage radiation beams used in External Beam Radiotherapy (EBRT) generate Cherenkov light emission in tissues and equivalent phantoms. This optical emission was utilized to excite an oxygen-sensitive phosphorescent probe, PtG4, which has been developed specifically for NIR lifetime-based sensing of the partial pressure of oxygen (pO2). Phosphorescence emission, at different time points with respect to the excitation pulse, was acquired by an intensifier-gated CCD camera synchronized with radiation pulses delivered by a medical linear accelerator. The pO2 distribution was tomographically recovered in a tissue-equivalent phantom during EBRT with multiple beams targeted from different angles at a tumor-like anomaly. The reconstructions were tested in two different phantoms that have fully oxygenated background, to compare a fully oxygenated and a fully deoxygenated inclusion. To simulate a realistic situation of EBRT, where the size and location of the tumor is well known, spatial information of a prescribed region was utilized in the recovery estimation. The phantom results show that region-averaged pO2 values were recovered successfully, differentiating aerated and deoxygenated inclusions. Finally, a simulation study was performed showing that pO2 in human brain tumors can be measured to within 15 mmHg for edge depths less than 10-20 mm using the Cherenkov Excited Phosphorescence Oxygen imaging (CEPhOx) method and PtG4 as a probe. This technique could allow non-invasive monitoring of pO2 in tumors during the normal process of EBRT, where beams are generally delivered from multiple angles or arcs during each treatment fraction.

Imaging tumor hypoxia to advance radiation oncology

SIGNIFICANCE

Most solid tumors contain regions of low oxygenation or hypoxia. Tumor hypoxia has been associated with a poor clinical outcome and plays a critical role in tumor radioresistance.

RECENT ADVANCES

Two main types of hypoxia exist in the tumor microenvironment: chronic and cycling hypoxia. Chronic hypoxia results from the limited diffusion distance of oxygen, and cycling hypoxia primarily results from the variation in microvessel red blood cell flux and temporary disturbances in perfusion. Chronic hypoxia may cause either tumor progression or regressive effects depending on the tumor model. However, there is a general trend toward the development of a more aggressive phenotype after cycling hypoxia. With advanced hypoxia imaging techniques, spatiotemporal characteristics of tumor hypoxia and the changes to the tumor microenvironment can be analyzed.

CRITICAL ISSUES

In this review, we focus on the biological and clinical consequences of chronic and cycling hypoxia on radiation treatment. We also discuss the advanced non-invasive imaging techniques that have been developed to detect and monitor tumor hypoxia in preclinical and clinical studies.

FUTURE DIRECTIONS

A better understanding of the mechanisms of tumor hypoxia with non-invasive imaging will provide a basis for improved radiation therapeutic practices.

Two-photon antenna-core oxygen probe with enhanced performance

Recent development of two-photon phosphorescence lifetime microscopy (2PLM) of oxygen enabled first noninvasive high-resolution measurements of tissue oxygenation in vivo in 3D, providing valuable physiological information. The so far developed two-photon-enhanced phosphorescent probes comprise antenna-core constructs, in which two-photon absorbing chromophores (antenna) capture and channel excitation energy to a phosphorescent core (metalloporphyrin) via intramolecular excitation energy transfer (EET). These probes allowed demonstration of the methods' potential; however, they suffer from a number of limitations, such as partial loss of emissivity to competing triplet state deactivation pathways (e.g., electron transfer) and suboptimal sensitivity to oxygen, thereby limiting spatial and temporal resolution of the method. Here we present a new probe, PtTCHP-C307, designed to overcome these limitations. The key improvements include significant increase in the phosphorescence quantum yield, higher efficiency of the antenna-core energy transfer, minimized quenching of the phosphorescence by electron transfer and increased signal dynamic range. For the same excitation flux, the new probe is able to produce up to 6-fold higher signal output than previously reported molecules. Performance of PtTCHP-C307 was demonstrated in vivo in pO2 measurements through the intact mouse skull into the bone marrow, where all blood cells are made from hematopoietic stem cells.

Optical methods for sensing and imaging oxygen: materials, spectroscopies and applications

We review the current state of optical methods for sensing oxygen. These have become powerful alternatives to electrochemical detection and in the process of replacing the Clark electrode in many fields. The article (with 694 references) is divided into main sections on direct spectroscopic sensing of oxygen, on absorptiometric and luminescent probes, on polymeric matrices and supports, on additives and related materials, on spectroscopic schemes for read-out and imaging, and on sensing formats (such as waveguide sensing, sensor arrays, multiple sensors and nanosensors). We finally discuss future trends and applications and summarize the properties of the most often used indicator probes and polymers. The ESI† (with 385 references) gives a selection of specific applications of such sensors in medicine, biology, marine and geosciences, intracellular sensing, aerodynamics, industry and biotechnology, among others.

Oxygen tomography by Čerenkov-excited phosphorescence during external beam irradiation

The efficacy of radiation therapy depends strongly on tumor oxygenation during irradiation. However, current techniques to measure this parameter in vivo do not facilitate routine monitoring in patients. Herein, we demonstrate a noninvasive method for tomographic imaging of oxygen partial pressure (pO(2)) in deep tissue using the phosphorescence decay of an oxygen-sensitive probe excited by Čerenkov radiation induced by external beam radiotherapy. Tissue-simulating scattering phantoms (60 mm diameter with a 20 mm anomaly) containing platinum(II)-G4 (PtG4), a dendritic porphyrin-based phosphor, whose phosphorescence is quenched in the presence of oxygen, were irradiated with a clinical linear accelerator. The emitted phosphorescence was measured at various positions on the phantom boundary using a spectrograph coupled to an intensified charge-coupled device (ICCD). At each position, PtG4 phosphorescence decay curves were measured by synchronizing the ICCD to the linear accelerator pulses. Tomographic images of phosphorescence yield and lifetime were recovered for phantoms with homogenous PtG4 concentrations and heterogeneous pO(2). Since PtG4 lifetime is strongly and predictably dependent on pO(2) through the Stern-Volmer relationship, tomographic images of pO(2) were also reported, and showed excellent agreement with independent oxygenation measurements. Translating this approach to the clinic could facilitate direct sensing of pO(2) during radiotherapy.

Modulation of Visible Room Temperature Phosphorescence by Weak Magnetic Fields

Magnetic control over excited states of molecules presents interest for many applications. Here we show for the first time that visible room temperature phosphorescence in multichromophoric donor-acceptor systems can be modulated by weak magnetic fields (<1 T) via magnetic field effects (MFE) on the spin dynamics in photogenerated radical pairs (RPs). The studied compounds comprise Pt porphyrin (PtP)-Rosamine B (RosB) dyads, which possess strong visible absorption bands and phosphoresce at room temperature. The observed MFE is unique in that it occurs upon direct excitation of the PtP in the dyads, whereby ultrafast quantitative formation of the local PtP triplet state precedes the occurrence of radical intermediates. A model explaining the effect is proposed, which is based on reversible electron transfer between the local triplet state and a long-lived RP. External magnetic field modulates spin dynamics in the RP, affecting contribution of the singlet RP recombination channel and thereby influencing phosphorescence.

Generalized coupled photon transport equations for handling correlated photon streams with distinct frequencies

A generalized form of coupled photon transport equations that can handle correlated light beams with distinct frequencies is introduced. The derivation is based on the principle of energy conservation. For a single frequency, the current formulation reduces to a standard photon transport equation, and for fluorescence and phosphorescence, the diffusion models derived from the proposed photon transport model match for homogenous media. The generalized photon transport model is extended to handle wideband inputs in the frequency domain.

Generation of phosphorescent triplet states via photoinduced electron transfer: energy and electron transfer dynamics in Pt porphyrin-Rhodamine B dyads

Control over generation and dynamics of excited electronic states is fundamental to their utilization in all areas of technology. We present the first example of multichromophoric systems in which emissive triplet states are generated via a pathway involving photoinduced electron transfer (ET), as opposed to local intrachromophoric processes. In model dyads, PtP-Ph(n)-pRhB(+) (1-3, n = 1-3), comprising platinum(II) meso-tetraarylporphyrin (PtP) and Rhodamine B piperazine derivative (pRhB(+)), linked by oligo-p-phenylene bridges (Ph(n)), upon selective excitation of pRhB(+) at a frequency below that of the lowest allowed transition of PtP, room-temperature T(1)→S(0) phosphorescence of PtP was observed. The pathway leading to the emissive PtP triplet state includes excitation of pRhB(+), ET with formation of the singlet radical pair, intersystem crossing within that pair, and subsequent radical recombination. Because of the close proximity of the triplet energy levels of PtP and pRhB(+), reversible triplet-triplet (TT) energy transfer between these states was observed in dyads 1 and 2. As a result, the phosphorescence of PtP was extended in time by the long decay of the pRhB(+) triplet. Observation of ET and TT in the same series of molecules enabled direct comparison of the distance attenuation factors β between these two closely related processes.

Design of Metalloporphyrin-Based Dendritic Nanoprobes for Two-Photon Microscopy of Oxygen

Metalloporphyrin-based phosphorescent nanoprobes are being developed for two-photon microscopy of oxygen. In these molecular constructs generation of porphyrin triplet states upon two-photon excitation is induced upon the intramolecular Förster-type resonance energy transfer from a covalently attached 2P antenna. In the earlier developed prototypes, electron transfer between the antenna and the metalloporphyrin strongly interfered with the phosphorescence, reducing the sensitivity and the dynamic range of the sensors. By tuning the distances between the antenna and the core and adjusting their redox potentials the unwanted electron transfer could be prevented. An array of phosphorescent Pt porphyrins (energy transfer acceptors) and 2P dyes (energy transfer donors) was screened using dynamic quenching of phosphorescence, and the FRET-pair with the minimal ET rate was identified. This pair, consisting of Coumarin-343 and Pt meso-tetra-(4-alkoxyphenyl)porphyrin, was used to construct a probe in which the antenna fragments were linked to the termini of G3 poly(arylglycine) (AG) dendrimer with PtP core. The folded dendrimer formed an insulating layer between the porphyrin and the antenna, simultaneously controlling the rate of oxygen quenching (Stern-Volmer oxygen quenching constant). Modification of the dendrimer periphery with oligoethyleneglycol residues made the probe's signal insensitive to the presence of proteins and other macromolecular solutes.

The dihydroisoindole approach to linearly annelated π-extended porphyrins

The dihydroisoindole strategy for the synthesis of linearly annelated π-extended porphyrins is a common approach to tetrabenzo-, tetranaphtho-, and tetraanthraporphyrins with variable functionality and substitution patterns. This method implies the use of a common type of precursors involving the 4,7-dihydroisoindole moiety, which are the closest possible stable relatives of the unstable isoindole, benzoisoindole and naphthoisoindole. A key feature of this strategy is the unprecedented mildness of the final oxidative aromatization step, which accounts for synthetic versatility, better functional group tolerance and high purity of the products. Many new linearly annelated π-extended porphyrins of the tetrabenzo-, tetranaphtho-, and tetraanthraporphyrin families were produced and characterized for the first time, including soluble planar and highly emissive 5,15-diaryl derivatives. The double bond of the dihydroisoindole fragment can also be useful for further modification of the porphyrin by means of addition of cycloaddition reactions, leading to new, previously inaccessible porphyins bearing multiple halide, hydroxy, or acetoxy groups at the periphery.

Using in-vivo fluorescence imaging in personalized cancer diagnostics and therapy, an image and treat paradigm

The major goal in developing drugs targeting specific tumor receptors, such as Monoclonal AntiBodies (MAB), is to make a drug compound that targets selectively the cancer-causing biomarkers, inhibits their functionality, and/or delivers the toxin specifically to the malignant cells. Recent advances in MABs show that their efficacy depends strongly on characterization of tumor biomarkers. Therefore, one of the main tasks in cancer diagnostics and treatment is to develop non-invasive in-vivo imaging techniques for detection of cancer biomarkers and monitoring their down regulation during the treatment. Such methods can potentially result in a new imaging and treatment paradigm for cancer therapy. In this article we have reviewed fluorescence imaging approaches, including those developed in our group, to detect and monitor Human Epidermal Growth Factor 2 (HER2) receptors before and during therapy. Transition of these techniques from the bench to bedside is the ultimate goal of our project. Similar approaches can be used potentially for characterization of other cancer related cell biomarkers.

Two new "protected" oxyphors for biological oximetry: properties and application in tumor imaging

We report the synthesis, calibration, and examples of application of two new phosphorescent probes, Oxyphor R4 and Oxyphor G4, optimized specifically for in vivo oxygen imaging by phosphorescence quenching. These "protected" dendritic probes can operate in either albumin-rich (blood plasma) or albumin-free (interstitial space) environments at all physiological oxygen concentrations, from normoxic to deep hypoxic conditions. Oxyphors R4 and G4 are derived from phosphorescent Pd-meso-tetra-(3,5-dicarboxyphenyl)-porphyrin (PdP) or Pd-meso-tetra-(3,5-dicarboxyphenyl)-tetrabenzoporphyrin (PdTBP), respectively, and possess features common for protected dendritic probes, i.e., hydrophobic dendritic encapsulation of phosphorescent metalloporphyrins and hydrophilic PEGylated periphery. The new Oxyphors are highly soluble in aqueous environments and do not permeate biological membranes. The probes were calibrated under physiological conditions (pH 6.4-7.8) and temperatures (22-38 °C), showing high stability, reproducibility of signals, and lack of interactions with biological solutes. Oxyphor G4 was used to dynamically image intravascular and interstitial oxygenation in murine tumors in vivo. The physiological relevance of the measurements was demonstrated by dynamically recording changes in tissue oxygenation during application of anesthesia (isofluorane). These experiments revealed that changes in isofluorane concentration significantly affect tissue oxygenation.

Evaluation of phototoxicity of dendritic porphyrin-based phosphorescent oxygen probes: an in vitro study

Biological oxygen measurements by phosphorescence quenching make use of exogenous phosphorescent probes, which are introduced directly into the medium of interest (e.g. blood or interstitial fluid) where they serve as molecular sensors for oxygen. The byproduct of the quenching reaction is singlet oxygen, a highly reactive species capable of damaging biological tissue. Consequently, potential probe phototoxicity is a concern for biological applications. Herein, we compared the ability of polyethyleneglycol (PEG)-coated Pd tetrabenzoporphyrin (PdTBP)-based dendritic nanoprobes of three successive generations to sensitize singlet oxygen. It was found that the size of the dendrimer has practically no effect on the singlet oxygen sensitization efficiency in spite of the strong attenuation of the triplet quenching rate with an increase in the dendrimer generation. This unexpected result is due to the fact that the lifetime of the PdTBP triplet state in the absence of oxygen increases with dendritic generation, thus compensating for the concomitant decrease in the rate of quenching. Nevertheless, in spite of their ability to sensitize singlet oxygen, the phosphorescent probes were found to be non-phototoxic when compared with the commonly used photodynamic drug Photofrin in a standard cell-survival assay. The lack of phototoxicity is presumably due to the inability of PEGylated probes to associate with cell surfaces and/or penetrate cellular membranes. In contrast, conventional photosensitizers bind to cell components and act by generating singlet oxygen inside or in the immediate vicinity of cellular organelles. Therefore, PEGylated dendritic probes are safe to use for tissue oxygen measurements as long as the light doses are less than or equal to those commonly employed in photodynamic therapy.

Tunable phosphorescent NIR oxygen indicators based on mixed benzo- and naphthoporphyrin complexes

A series of π-extended phosphorescent palladium(II) and platinum(II) porphyrin complexes were synthesized, in which additional benzene rings are fused radially onto at least one of the four peripheral benzo groups. The photophysical properties of the metalloporphyrins palladium(II)-meso-tetra-(4-fluorophenyl)mononaphthotribenzoporphyrin (Pd1NF), cis-palladium(II)-meso-tetra-(4-fluorophenyl)dibenzodinaphthoporphyrin (Pd2NF), and palladium(II)-meso-tetra-(4-fluorophenyl)monobenzotrinaphthoporphyrin (Pd3NF) and the corresponding platinum(II) compounds (Pt1NF, cis-Pt2NF, Pt3NF) were investigated. The compounds under investigation absorb intensively in the near-infrared region (628-691 nm) and emit at room temperature at 815-882 nm. Phosphorescence quantum yields of the platinum(II) porphyrins range from 25 to 53% with luminescence decay times of 21 to 44 μs in deoxygenated toluene solutions at room temperature. The corresponding palladium(II) complexes exhibit quantum yields in the range of 7 to 18% with lifetimes of 106 to 206 μs. Density functional theory (DFT) calculations revealed nonplanar geometries for all complexes and corroborate the absorption characteristics. The subsequent π extension of the porphyrin system leads to near-infrared absorbing oxygen indicators with tailor-made luminescence properties as well as tunable oxygen sensitivity.

Optical oxygen sensors for applications in microfluidic cell culture

The presence and concentration of oxygen in biological systems has a large impact on the behavior and viability of many types of cells, including the differentiation of stem cells or the growth of tumor cells. As a result, the integration of oxygen sensors within cell culture environments presents a powerful tool for quantifying the effects of oxygen concentrations on cell behavior, cell viability, and drug effectiveness. Because microfluidic cell culture environments are a promising alternative to traditional cell culture platforms, there is recent interest in integrating oxygen-sensing mechanisms with microfluidics for cell culture applications. Optical, luminescence-based oxygen sensors, in particular, show great promise in their ability to be integrated with microfluidics and cell culture systems. These sensors can be highly sensitive and do not consume oxygen or generate toxic byproducts in their sensing process. This paper presents a review of previously proposed optical oxygen sensor types, materials and formats most applicable to microfluidic cell culture, and analyzes their suitability for this and other in vitro applications.

Diffuse Optics for Tissue Monitoring and Tomography

This review describes the diffusion model for light transport in tissues and the medical applications of diffuse light. Diffuse optics is particularly useful for measurement of tissue hemodynamics, wherein quantitative assessment of oxy- and deoxy-hemoglobin concentrations and blood flow are desired. The theoretical basis for near-infrared or diffuse optical spectroscopy (NIRS or DOS, respectively) is developed, and the basic elements of diffuse optical tomography (DOT) are outlined. We also discuss diffuse correlation spectroscopy (DCS), a technique whereby temporal correlation functions of diffusing light are transported through tissue and are used to measure blood flow. Essential instrumentation is described, and representative brain and breast functional imaging and monitoring results illustrate the workings of these new tissue diagnostics.

Neutrophil alpha-defensins cause lung injury by disrupting the capillary-epithelial barrier

RATIONALE

The involvement of neutrophil activation in the sentinel, potentially reversible, events in the pathogenesis of acute lung injury (ALI) is only partially understood. alpha-Defensins are the most abundant proteins secreted by activated human neutrophils, but their contribution to ALI in mouse models is hindered by their absence from murine neutrophils and the inability to study their effects in isolation in other species.

OBJECTIVES

To study the role of alpha-defensins in the pathogenesis of ALI in a clinically relevant setting using mice transgenic for polymorphonuclear leukocyte expression of alpha-defensins.

METHODS

Transgenic mice expressing polymorphonuclear leukocyte alpha-defensins were generated. ALI was induced by acid aspiration. Pulmonary vascular permeability was studied in vivo using labeled dextran and fibrin deposition. The role of the low-density lipoprotein-related receptor (LRP) in permeability was examined.

MEASUREMENTS AND MAIN RESULTS

Acid aspiration induced neutrophil migration and release of alpha-defensins into lung parenchyma and airways. ALI was more severe in alpha-defensin-expressing mice than in wild-type mice, as determined by inspection, influx of neutrophils into the interstitial space and airways, histological evidence of epithelial injury, interstitial edema, extravascular fibrin deposition, impaired oxygenation, and reduced survival. Within 4 hours of insult, alpha-defensin-expressing mice showed greater disruption of capillary-epithelial barrier function and ALI that was attenuated by systemic or intratracheal administration of specific inhibitors of the LRP.

CONCLUSIONS

alpha-Defensins mediate ALI through LRP-mediated loss of capillary-epithelial barrier function, suggesting a potential new approach to intervention.

Near-infrared phosphorescent polymeric nanomicelles: efficient optical probes for tumor imaging and detection

We report a formulation of near-infrared (near-IR) phosphorescent polymeric nanomicelles and their use for in vivo high-contrast optical imaging, targeting, and detection of tumors in small animals. Near-IR phosphorescent molecules of Pt(II)-tetraphenyltetranaphthoporphyrin (Pt(TPNP)) were found to maintain their near-IR phosphorescence properties when encapsulated into phospholipid nanomicelles. The prepared phosphorescent micelles are of approximately 100 nm size and are highly stable in aqueous suspensions. A large spectral separation between the Pt(TPNP) absorption, with a peak at approximately 700 nm, and its phosphorescence emission, with a peak at approximately 900 nm, allows a dramatic decrease in the level of background autofluorescence and scattered excitation light in the near-IR spectral range, where the signal from the phosphorescent probe is observed. In vivo animal imaging with subcutaneously xenografted tumor-bearing mice has resulted in high contrast optical images, indicating highly specific accumulation of the phosphorescent micelles into tumors. Using optical imaging with near-IR phosphorescent nanomicelles, detection of smaller, visually undetectable tumors has also been demonstrated.

Dendritic phosphorescent probes for oxygen imaging in biological systems

Oxygen levels in biological systems can be measured by the phosphorescence quenching method using probes with controllable quenching parameters and defined biodistributions. We describe a general approach to the construction of phosphorescent nanosensors with tunable spectral characteristics, variable degrees of quenching, and a high selectivity for oxygen. The probes are based on bright phosphorescent Pt and Pd complexes of porphyrins and symmetrically pi-extended porphyrins (tetrabenzoporphyrins and tetranaphthoporphyrins). pi-Extension of the core macrocycle allows tuning of the spectral parameters of the probes in order to meet the requirements of a particular imaging application (e.g., oxygen tomography versus planar microscopic imaging). Metalloporphyrins are encapsulated into poly(arylglycine) dendrimers, which fold in aqueous environments and create diffusion barriers for oxygen, making it possible to regulate the sensitivity and the dynamic range of the method. The periphery of the dendrimers is modified with poly(ethylene glycol) residues, which enhance the probe's solubility, diminish toxicity, and help prevent interactions of the probes with the biological environment. The probe's parameters were measured under physiological conditions and shown to be unaffected by the presence of biomacromolecules. The performance of the probes was demonstrated in applications, including in vivo microscopy of vascular pO(2) in the rat brain.

Effects of structural deformations on optical properties of tetrabenzoporphyrins: free-bases and Pd complexes

A recently developed method of synthesis of pi-extended porphyrins made it possible to prepare a series of tetrabenzoporphyrins (TBP) with different numbers of meso-aryl substituents. The photophysical parameters of free-bases and Pd complexes of meso-unsubstituted TBP's, 5,15-diaryl-TBP's (Ar2TBP's) and 5,10,15,20-tetraaryl-TBP's (Ar4TBP's) were measured. For comparison, similarly meso-arylsubstituted porphyrins fused with nonaromatic cyclohexeno-rings, i.e. Ar(n)-tetracyclohexenoporphyrins (Ar(n)TCHP's, n = 0, 2, 4), were also synthesized and studied. Structural information was obtained by ab initio (DFT) calculations and X-ray crystallography. It was found that: 1) Free-base Ar4TBP's are strongly distorted out-of-plane (saddled), possess broadened, red-shifted spectra, short excited-state lifetimes and low fluorescence quantum yields (tau(fl) = 2-3 ns, phi(fl) = 0.02-0.03). These features are characteristic of other nonplanar free-base porphyrins, including Ar4TCHP's. 2) Ar2TBP free-bases possess completely planar geometries, although with significant in-plane deformations. These deformations have practically no effect on the singlet excited-state properties of Ar2TBP's as compared to planar meso-unsubstituted TBP's. Both types of porphyrins retain strong fluorescence (tau(fl) = 10-12 ns, phi(fl) = 0.3-0.4), and their radiative rate constants (k(r)) are 3-4 times higher than those of planar H2TCHP's. 3) Nonplanar deformations dramatically enhance nonradiative decay of triplet states of regular Pd porphyrins. For example, planar PdTCHP phosphoresces with high quantum yield (phi(phos) = 0.45, tau(phos) = 1118 micros), while saddled PdPh4TCHP is practically nonemissive. In contrast, both ruffled and saddled PdAr(n)TBP's retain strong phosphorescence at ambient temperatures (PdPh2TBP: tau(phos) = 496 micros, phi(phos) = 0.15; PdPh4TBP: tau(phos) = 258 micros, phi(phos) = 0.08). It appears that pi-extension is capable of counterbalancing deleterious effects of nonplanar deformations on triplet emissivity of Pd porphyrins.

Synthesis of 5,15-diaryltetrabenzoporphyrins

A general method of synthesis of 5,15-diaryltetrabenzoporphyrins (Ar 2TBPs) has been developed, based on 2 + 2 condensation of dipyrromethanes followed by oxidative aromatization. Two pathways to Ar 2TBPs were investigated: the tetrahydroisoindole pathway and the dihydroisoindole pathway. In the tetrahydroisoindole pathway, precursor 5,15-diaryltetracyclohexenoporphyrins (5,15-Ar 2TCHPs) were assembled from cyclohexeno-fused meso-unsubstituted dipyrromethanes and aromatic aldehydes or, alternatively, by way of the classical MacDonald synthesis. In the first case, scrambling was observed. Aromatization by tetracyclone was more effective than aromatization by DDQ but failed in the cases of porphyrins with electron-withdrawing substituents in the meso-aryl rings. The dihydroisoindole pathway was found to be much superior to the tetrahydroisoindole pathway, and it was developed into a general preparative method, consisting of (1) the synthesis of 4,7-dihydroisoindole and its transformation into meso-unsubstituted dipyrromethanes, (2) the synthesis of 5,15-diaryloctahydrotetrabenzoporphyrins (5,15-Ar 2OHTBPs), and (3) their subsequent aromatization by DDQ. Ar 2TBP free bases exhibit optical absorption spectra similar to those of meso-unsubstituted tetrabenzoporphyrins and fluoresce with high quantum yields. Pd complex of Ph 2TBP was found to be highly phosphorescent at room temperature.

Influence of optical heterogeneities on reconstruction of spatial phosphorescence lifetime distributions

We have previously demonstrated that phosphorescence lifetime imaging (PLI) allows for unbiased determination of absolute oxygen levels in homogeneously absorbing/scattering media. We computationally show that tomographic PLI can perform equally well in heterogeneous environments relying only on surface measurements of phosphorescence.

Fluorescence lifetime imaging by using time-gated data acquisition

The use of the time gating technique for lifetime reconstruction in the Fourier domain is a novel technique. Time gating provides sufficient data points in the time domain for reliable application of the Fourier transform, which is essential for the time deconvolution of the system of the integral equations employed in the reconstruction. The Fourier domain telegraph equation is employed to model the light transport, which allows a sufficiently broad interval of frequencies to be covered. Reconstructed images contain enough information needed for recovering the lifetime distribution in a sample for any given frequency within the megahertz-gigahertz band. The use of this technique is essential for recovering time-dependent information in fluorescence imaging. This technique was applied in reconstruction of the lifetime distribution of four tubes filled with Rhodamine 6G embedded inside a highly scattering slab. Relatively accurate fluorescence lifetime reconstruction demonstrates the effectiveness and the potential of the proposed technique.

Energy and electron transfer in enhanced two-photon-absorbing systems with triplet cores

Enhanced two-photon-absorbing (2PA) systems with triplet cores are currently under scrutiny for several biomedical applications, including photodynamic therapy (PDT) and two-photon microscopy of oxygen. The performance of so far developed molecules, however, is substantially below expected. In this study we take a detailed look at the processes occurring in these systems and propose ways to improve their performance. We focus on the interchromophore distance tuning as a means for optimization of two-photon sensors for oxygen. In these constructs, energy transfer from several 2PA chromophores is used to enhance the effective 2PA cross section of phosphorescent metalloporphyrins. Previous studies have indicated that intramolecular electron transfer (ET) can act as an effective quencher of phosphorescence, decreasing the overall sensor efficiency. We studied the interplay between 2PA, energy transfer, electron transfer, and phosphorescence emission using Rhodamine B-Pt tetrabenzoporphyrin (RhB-PtTBP) adducts as model compounds. 2PA cross sections (sigma2) of tetrabenzoporphyrins (TBPs) are in the range of several tens of GM units (near 800 nm), making TBPs superior 2PA chromophores compared to regular porphyrins (sigma2 values typically 1-2 GM). Relatively large 2PA cross sections of rhodamines (about 200 GM in 800-850 nm range) and their high photostabilities make them good candidates as 2PA antennae. Fluorescence of Rhodamine B (lambda(fl) = 590 nm, phi(fl) = 0.5 in EtOH) overlaps with the Q-band of phosphorescent PtTBP (lambda(abs) = 615 nm, epsilon = 98 000 M(-1) cm(-1), phi(p) approximately 0.1), suggesting that a significant amplification of the 2PA-induced phosphorescence via fluorescence resonance energy transfer (FRET) might occur. However, most of the excitation energy in RhB-PtTBP assemblies is consumed in several intramolecular ET processes. By installing rigid nonconducting decaproline spacers (Pro10) between RhB and PtTBP, the intramolecular ETs were suppressed, while the chromophores were kept within the Förster r0 distance in order to maintain high FRET efficiency. The resulting assemblies exhibit linear amplification of their 2PA-induced phosphorescence upon increase in the number of 2PA antenna chromophores and show high oxygen sensitivity. We also have found that PtTBPs possess unexpectedly strong forbidden S0 --> T1 bands (lambda(max) = 762 nm, epsilon = 120 M-1 cm-1). The latter may overlap with the laser spectrum and lead to unwanted linear excitation.